It is known in the art to utilize positive displacement compressors having lobed rotors for supercharging internal combustion engines and for providing compressed air for other purposes. Such a compressor used as an automotive supercharger may include a housing having a rotor cavity in which a pair of parallel rotors having interleaved lobes rotate to effect compression of air discharged through an opening in the cavity wall near an outlet end of the housing. The rotors may be belt driven by the engine through a pulley connected directly, or through a gear train, to the pair of rotors. A closure may be mounted on the housing to contain the compressed air before discharging it air through the housing or otherwise to the engine air intake and associated cylinders.
Between the housing and the closure, an outlet plenum is formed which receives the compressed air from the rotors through an outlet near an outlet end of the cavity. The plenum is subject to pulsations created by the pumping action of the rotors. Accordingly, the outlet side of the housing wall is desirably stiffened by providing a cross-ribbed pattern or grid similar to a waffle. The ribs provide a stiffened portion to reduce flexing of the rotor wall. The waffle pattern results in a plurality of lightening recesses in the wall to reduce its mass and the weight of the housing.
The pulsations created in the plenum react with the air induction system of the engine, particularly within the rotor housing an plenum, to cause vibrations of the structure that result in undesired noise of various frequencies, which it is desired to minimize.